Sizing a Boiler Expansion Tank

I have a new condensing boiler with a 30 gallon buffer tank and need help sizing an expansion tank since I'm having problems. Here's the info:

Buffer Tank = 30 Gallons
Boiler and Piping = 10.6 gallons

Total water = 40.6 gallons

Min Boiler Temp = 80F
Max Boiler Temp = 180F

Operating Boiler Pressure = 12PSI

I have a Extrol #30 4.4 Gallon, but the pressure is always above 15PSI and sometime 24 PSI. I'm thinking I need the #60 (7.6 Gallons) since I added the 30 Gallon Buffer Tank which is new. Not sure if this is the problem or it's the PRV, but the addition of 30 Gallons may have put me into the the next size... I checked the system with te main shut off so the PRV wa basically off and the pressure still raised from 12 to 16..

12 psi is the cold start pressure. As water heats it expands and presses against the precharged bladder. At full temp of 180 degrees the system will normally be in the low 20's in pressure.
A bigger tank may drop the upper end 2 PSI at most, but not needed.

All is good with your system. Don't touch a thing.

If the pressure didn't increase when heated, you would experience flashing and pump cavitation that you don't want.

Using the calculator on this page: http://www.watts.com/pages/support/sizing_ET.asp , you need a tank with at least 3.36 total gallons (from 70-190 degrees, the water will expand 1.21g). So, you should be okay. Put your own numbers in, then compare the volumes required with the spec sheet of what you have to confirm.

And, depending on where the boiler is mounted, and where the burner is, and considering the upper most floor's radiators are just that, near the floor, you may not have 35' of rise. Now, if you have 10' ceilings in your old house, maybe...

Thanks guys - One last question... If the system pressure starts cold at 12psi and goes up to 18psi or maybe 21psi once the boiler runs for a while, should the pressure reduce back to 12psi when the system is off for awhile and cold, or is it OK for the system to return to only about 16 psi?